1. Field of the Invention
The invention relates to an amplifier circuit of a capacitor microphone generating an electric signal corresponding to sound.
2. Description of the Related Art
There are a MEMS microphone and an electret capacitor microphone (ECM) as a type of capacitor microphone. The fundamental structure of the MEMS microphone is a capacitor made of two electrode plates, i.e., a diaphragm and a back plate that are closely faced and disposed, and this structure is formed on a silicon substrate by a MEMS (Micro Electro Mechanical Systems) technique. This MEMS microphone is resistant to a temperature in a normal solder reflow process, and may be soldered on a printed board together with other components, for example. The MEMS microphone may be formed smaller than a general electret capacitor microphone (ECM). For this reason, a device with the MEMS microphone achieves high packaging density and miniaturization.
While the ECM uses an electret element that stores electric charge semipermanently and thus does not need a bias voltage, the MEMS microphone needs a relatively high direct current bias voltage for operation. By applying this bias voltage, a constant electric charge Q is charged in the capacitor forming the MEMS microphone. When the diaphragm vibrates by sound pressure in this state, the capacitance C of the capacitor changes to change a voltage V between terminals. This change of the voltage V is outputted as a sound signal.
An amplifier circuit of a MEMS microphone is described in Japanese Patent Application Publication No. 2008-153981, as formed using a charge pump circuit as a bias voltage generation source and an operational amplifier as a signal amplification portion. An amplifier circuit of an electret capacitor microphone (ECM) is described in Japanese Patent Application Publication No. 2001-102875.
In an amplifier circuit of a capacitor microphone, an inversion amplifier circuit is formed using an operational amplifier, a capacitor microphone Cm and a feedback capacitor Cf. An input signal from the capacitor microphone is amplified by a ratio of CAm/CAf and outputted, where CAm is the capacitance value of the capacitor microphone Cm, and CAf is the capacitance value of the feedback capacitor Cf. In this case, when the input signal inputted to the inversion amplifier circuit is too high, there occurs a problem such that an output signal of the inversion amplifier circuit has an amplitude at the full scale level of a circuit in a subsequent stage or more or becomes the supply voltage of the inversion amplifier or more, and is clipped at the supply voltage (i.e., waveform distortion occurs).
Particularly when the circuit in the subsequent stage that receives the output signal of the inversion amplifier circuit is an AD converter (an analog-digital converter), the level of the output signal of the inversion amplifier circuit has an amplitude at the full scale level of the AD converter or more (0 dBFS or more), or the waveform of the output signal becomes distorted. Then there occurs a problem such that the level of the output signal of the AD converter is saturated (clipped) and the total harmonic distortion characteristic (THD characteristic) is highly enhanced.